Oscillatory signals are generated for example in a receiver which receives a travelling wave such as sound or an electromagnetic wave, e.g. a radio wave, and produces an output, e.g. a voltage or current, in response to the instantaneous amplitude of the received wave. Conversely, an oscillatory signal may be generated on purpose in order to generate a corresponding wave to be emitted by e.g. a loudspeaker or an antenna.
A signal often has a well-defined oscillation frequency, also referred to as the carrier frequency. Information can be modulated onto such a signal (carrier signal or carrier) by modulating e.g. its amplitude (amplitude modulation, AM) or its frequency (frequency modulation, FM). The information modulated on the carrier signal may for example be an analogous audio signal or a digital bit stream. For example, using FM a succession of binary bits can be modulated onto the carrier by varying the carrier frequency between a first frequency F0 representing a logical ZERO (LOW) and a second frequency F1 representing a logical ONE (HIGH). Alternatively, using AM the carrier's amplitude may be varied between a first amplitude A0 representing the logical ZERO and a second amplitude A1 representing the logical ONE. Both A1 and A2 may be non-vanishing amplitudes. Alternatively, one of these amplitudes, e.g. A1, is non-vanishing while the other amplitude, e.g. A0, is zero. In this case, the carrier signal is a succession of pulses, each pulse having a constant frequency and constant amplitude. Each pulse thus represents a succession of one or more bits in the ONE state, while each gap between consecutive pulses represents a succession of one or more bits in the ZERO state.
Signal detectors are generally used to detect automatically a signal of interest in a received signal. The received signal may comprise undesired signals and noise instead or in addition to the signal of interest. Upon detecting that the signal of interest is present in the received signal, a specific action may be triggered, such as further processing the received signal, switching from a current carrier to a newly detected carrier, and/or energizing a device for processing the signal of interest. In a typical application, a receiver is periodically turned on to listen if another device attempts to communicate with it. Its turn-on time (T_ON) is often required to be as short as possible because it may directly define the energy consumption of the system as T_ON/T_OFF times the receivers power. In particular in short-range devices the energy consumption is often a key parameter and should be kept as low as possible.
Various methods for detecting a carrier signal are known in the art. For example, some receivers try to identify a specific key encoded in the received signal, or measure the received signal strength to provide a received signal strength indication (RSSI). The first option (identifying a specific key) usually requires a long turn-on time, whereas the second option (measuring the received signal strength) requires a high accuracy in the power measurement which in turn requires a tight receive gain control.
U.S. Pat. No. 5,111,479 describes a carrier sense circuit for sensing a carrier component of a received signal in a spread spectrum system. The circuit includes a multiplier which produces a squared signal by multiplying the received signal by itself, a filter, and a detecting circuit.
JP 2008/85669 discloses a carrier sensing circuit comprising a mixer for mixing a received radio frequency signal and a local signal, and a band-pass filter.
JP 04 216226 describes a received signal detector which determines a reception level, while JP 04 103230 presents a carrier sense circuit based on RSSI.